Induction of a T lymphocyte response is a critical initial step in a host's immune response. Activation of T cells results in cytokine production by T cells, T cell proliferation, and generation of T-cell-mediated effector functions.
The cytokines are a diverse group of structurally dissimilar and genetically unrelated molecules. Cytokines serve as crucial intercellular-signaling molecules that are responsible for the multidirectional communication among immune and inflammatory cells engaged in host defense, repair, and restoration of homeostasis, as well as among other somatic cells in the connective tissues, skin, nervous system, and other organs. More particularly, this diverse group of intercellular-signaling proteins regulates local and systemic immune and inflammatory responses as well as wound healing, hematopoiesis, and many other biological processes.
Each cytokine is secreted by particular cell types in response to a variety of stimuli and produces a characteristic constellation of effects on the growth, motility, differentiation, or function of its target cells. In fact, cytokines regulate one another's production and activities. Other types of biological mediators, such as corticosteroids and prostaglandins, have agonistic or antagonistic effects on cytokine activities.
Interleukin-2 (IL-2) is an autocrine and paracrine growth factor that is secreted by activated T lymphocytes. IL-2 is a critical immunoregulatory cytokine as it is essential for clonal T-cell proliferation, is involved in cytokine production, and influences the functional properties of B cells, macrophages, and NK cells. IL-2 enhances proliferation and antibody secretion by normal B cells. However, the concentration required for the B-cell response is two- to three-fold higher than is required to obtain T-cell responses. Higher concentrations of IL-2 can also activate neutrophils. IL-2 exhibits a short half-life in the circulation. Thus, it generally acts only on the cell that secreted it or on cells in the immediate vicinity.
The IL-2 receptor is not expressed in resting T cells but is induced to maximal levels within two or three days after the cells become activated. A decline in receptor expression occurs up to 6-10 days after activation. This transient nature of IL-2 receptor expression maintains the cyclical, self-limiting pattern of normal T-cell growth in vivo.
During the course of an immune response, T cells differentiate into Th phenotypes defined by their pattern of cytokine secretion and immunomodulatory properties (Abbas et al. (1996) Nature 383:787). Th cells are composed of at least two distinct subpopulations, termed Th1 and Th2 cell subpopulations (Mosmann et al. (1989) Ann. Rev. Immunol. 7:145; Del Prete et al. (1991) J. Clin. Invest. 88:346; Wiernenga et al. (1990) J. Immunol. 144:4651; Yamamura et al. (1991) Science 254:277; Robinson et al. (1993) J. Allergy Clin. Immunol. 92:313). Th1 and Th2 cells appear to function as part of the different effector functions of the immune system (Mosmann et al. (1989) Ann. Rev. Immunol. 7:145). Specifically, Th1 cells direct the development of cell-mediated immunity, triggering phagocyte-mediated host defenses, and are associated with delayed hypersensitivity. Accordingly, infections with intracellular microbes tend to induce Th1-type responses. Th2 cells drive humoral immune responses, which are associated with, for example, defenses against certain helminthic parasites, and are involved in antibody and allergic responses.
Th1 cells secrete interleukin-2 (IL-2), interferon-γ (IFN-γ), and tumor neucrosis factor-α (TNF-α). These cytokines enhance inflammatory cell-mediated responses and have a pathogenic role in the development of autoimmune disease. Th2 cells secrete interleukin-4 (IL-4), interleukin-5 (IL-5), interleukin-10 (IL-10), and interleukin-13 (IL-13). These cytokines suppress inflammatory responses while potentiating humoral immunity and control and reverse disease evolution (Scott et al. (1994) Immunity 1:73; Smith et al. (1998) J. Immunol. 160:4841; Abbas et al. (1996) Nature 383:787). The different type of cytokines released upon stimulation has been demonstrated to be central to disease evolution (Chu and Londei (1996) J. Immunol. 157:2685; Hsieh et al. (1993) Science 260:547).
T-cell activation requires two signals. The first is an antigen-specific signal, often called a primary activation signal, which results from stimulation of a T-cell receptor present on the surface of the T cell. This antigen-specific signal is usually in the form of an antigenic peptide bound either to a major histocompatibility complex (hereafter MHC) class I protein or an MHC class II protein present on the surface of an antigen presenting cell (hereafter APC). For a review see Germain (1986) Nature 322:687-691.
In addition to an antigen-specific primary activation signal, T cells also require a second, non-antigen specific signal, to induce T-cell proliferation and/or cytokine production. This phenomenon has been termed co-stimulation (Mueller et al. (1989) Annu. Rev. Immunol. 7:445-480). This “two signal” concept explains why adaptive immunity is elicited by microbes and not by self-antigens, which do not induce second signals.
Like the antigen-specific signal, the co-stimulatory signal is triggered by a molecule on the surface of the antigen presenting cell (APC). The B7 molecules are an emerging family of immunoglobulin co-stimulatory molecules, first identified on B lymphocytes (Linsley et al. (1990) Proc. Natl. Acad. Sci. 87:5031-5035). Both B7-1 (CD80) and B7-2 (CD86) bind to the T cell receptors CD28 and CTLA4, resulting in co-stimulation of the T cell (Peach et al. (1995) J. Biol. Chem. 270:21181-21187; Fargeas et al. (1995) J. Exp. Med. 182:667-675; Bajorath et al. (1994) Protein Sci. 3:2148-2150; U.S. Pat. No. 5,942,607; and PCT Application No. WO 96/40915). Depending upon which receptor is bound, the activated T-cell immune response is enhanced (CD28) or inhibited (CTLA4) in a negative feedback loop. Additional B7 homologs have been identified including B7-H1, and B7RP-1 and its mouse ortholog B7h (Swallow et al. (1999) Immunity 11:423-432; Dong et al. (1999) Nature Med. 5:1365-1369; Yoshinaga et al. (1999) Nature 402:827-832). Although both B7RP-1 and B7-H1 co-stimulate T-cell proliferation, neither of these molecules binds to either CD28 or CTLA4 (Abbas and Sharpe (1999) Nature Med. 5:1345-1346; Yoshinaga et al. (1999) Nature 402:827-832). Unlike B7-1 and B7-2, B7-H1 has little effect on IL-2 production, but considerably increases T-cell production of IL-10, a B-cell differentiation factor that inhibits macrophages and cell-mediated immunity.
Ligation of the CD28 family member ICOS (inducible co-stimulator) increases IL-10 production. B7RP-1 has been shown to bind to this receptor (Yoshinaga et al. (1999) Nature 402:827-832) while B7-H1 does not appear to bind to ICOS (Dong et al. (1999) Nature Med. 5:1365-1369), although this result is not definitive. Like CD28, ICOS enhances all basic T-cell responses to a foreign antigen, namely, proliferation, secretion of lymphokines, up-regulation of molecules that mediate cell-cell interaction, and effective help for antibody secretion by B-cells. Unlike the constitutively expressed CD28, ICOS has to be de novo induced on the T-cell surface, does not up-regulate the production of IL-2, but superinduces the synthesis of IL-10 (Hutloff et al. (1999) Nature 397:263-266). The inducible expression of ICOS shortly after T-cell activation indicates that ICOS may be particularly important in providing co-stimulatory signals to activated T cells, in contrast to CD28, which is essential in the activation and differentiation of naïve T cells (McAdam et al. (1998) Immunol. Rev. 165:231-247). ICOS may down-regulate immune responses by stimulating development of regulatory T cells, which normally function to control the injurious side effects of cell-mediated immunity. As ICOS signaling induces IL-10, which can also down-regulate B7-1 and B7-2 expression (Ding et al. (1993) J. Immunol. 151:1224-1234), ICOS co-stimulation may indirectly reduce or inhibit B7 expression and thereby inhibit B7-mediated CD28 co-stimulation. Therefore, whereas B7-1 and B7-2 function in the initiation and development of immune responses, B7RP-1 and B7-H1 may function to return the immune system to its resting state.
Another receptor belonging to the immunoglobulin gene superfamily, designated PD-1, also appears to be involved in the negative regulation of certain immune responses. PD-1 knockout mice develop Lupus-like autoimmune diseases (Nishimura et al. (1999) Immunity 11:141-151). In addition, the identification of a novel member of the B7 family (PD-L) that binds to the PD-1 receptor but not CD28, CTLA4, or ICOS has been reported (Freeman et al. (2000) FASEB J. 14(6):Abstract 153.34).
The profile of the natural immune response, specifically cytokine production, may determine the phenotype of the subsequent immune response. Therefore, methods are needed to regulate an immune response. There is great interest in the possibility that in disease situations in which antigens are either unknown or difficult to manipulate, immune responses may be either enhanced or terminated by manipulating the co-stimulation signals such as those signals affected by the B7 family of proteins. For example, modulating the co-stimulation signals may promote tumor immunity and reduce graft rejection, autoimmune, inflammatory, and infectious diseases (Abbas and Sharpe (1999) Nature Med. 5:1345-1346; Schweiter and Sharpe (1998) J. Immunol. 161:2762-2771; Wallace et al. (1994) Transplantation 58:602; Sayegh (1995) J. Exp. Med. 181:1869; Lenschow et al. (1995)J. Exp. Med. 181:1145; Fincket al. (1994) Science 265:1225; Cross et al. (1995) J. Clin. Invest. 95:2783; Perrin et al. (1995) J. Immunol. 154:1481; Corry et al. (1994)J. Immunol. 153:4142; U.S. Pat. Nos. 5,968,510, 5,861,310, and 5,521,288; and PCT Application No. WO 90/05541 and European Patent No. EP445228B1).